This invention relates to a system for and a method of multiplexing speech signals and in particular to a digital speech interpolation (DSI) technique for multiplexing speech signals with a high efficiency and to a system for and a method of multiplexing speech signals in which control of the amount of speech information to be multiplexed is possible.
An effective speech signal multiplexing system has been proposed in which many terminals are connected with transmission channels, the number of the terminals being greater than that of the channels. In such a system, when communication requests are made simultaneously from a number of terminals, which is greater than the number of channels stated above, communication above the capacity of the channels is made possible by using the channels in time division fashion while intercepting appropriately speech frames having small influences on the quality of speech sound. As an example of this kind of a system, a multiplexing system is disclosed in JP-A-61-77439 (laid-open on Apr. 21, 1986) in which, exclusion or "freeze-out" in particular channels has been controlled by regulating the discrimination level and the hangover time of the speech detector, using past statistical data. Here the principal item of the past statistical data is the past rate of freeze-out of speech in the particular channels. The other channels have no influence on the statistical data and many other items of the statistical data are not used. Further, in the case where the number of communication requests is greater than the number of channels, the amount of speech information judged to be talk-spurt is reduced by raising the discrimination level of the speech detector so that speech information lower than the raised level is judged not to be speech. Furthermore, for the hangover time, i.e. time sections such as at the termination of words or in intervals between words where the speech information level is low, if the level of the detector is set high, since the information during such time sections are cut or neglected, they have influences on the speech sound quality. However, it is possible to deal with them as speech and to reduce the number of cuts by adding superfluous time thereto. Consequently, the amount of information judged to be talk-spurt can be controlled by regulating the hangover time added to the termination or the interval.
In this way, according to the prior art techniques the amount of the speech information is reduced by regulating the detection level and the hangover time. However, according to the techniques, since the transmission control of each of the other channels is performed by using the statistical data within a particular channel, there is no guarantee that the speech information having the smallest influences on the speech sound quality among all the channels is frozen-out. For example, even in the case where there existed important speech information in a certain channel A and speech information having small influences on the speech sound quality such as noise in another channel B, according to the prior art techniques, the "importance (likelihood of being speech)" is not compared between A and B. Therefore, the channels A and B will be controlled in the same way and thus speech information is cut down similarly for both the channels. Thus, it is desirable not to control important speech and noise in the same manner, but to freeze-out only speech having practically small influences on the speech sound quality.